Certain applications may require nondestructive inspection evaluation (NDE) of new or used fracture-critical and/or failure-critical components. For example, in space-based and certain aeronautical applications, there may be elevated concern relating to the use of certain components due to aging and/or impact damage of the components. The presence of one-of-a-kind or few-of-a-kind critical components having a limited inspection history and use, and/or that are constructed of materials having limited availability, has only enhanced the overall inspection concern.
The determination of the capability of conventional inspection systems and methodologies using curve fitting or other techniques may be insufficient for use with updated and rapidly changing inspection requirements for such systems. For example, the National Aeronautics and Space Administration (NASA) currently requires on-orbit inspections of the Space Shuttle Orbiter's external thermal protection system. On-orbit testing is typically performed by trained astronauts as an extravehicular activity (EVA). Inspection of fracture-critical and failure-critical components requires inspection to be at 90% probability of detection (POD) with a 95% level of confidence, commonly referred to in the art and herein as a 90/95 POD.
Design of experiments or DOE describes a statistics-based process in which changes are made to various input variables of a system, and the effects on response variables are measured and recorded. DOE may utilize the concept of “point estimate probability of a hit” or POH at a given “flaw” size, with the term “flaw” referring to a physical flaw such as a crack in a component when used with physical inspection systems. When used with other systems, the term “flaw” may refer to any other variable one wishes to inspect for, e.g., delivery times, flavor levels in a food product, engineering properties, etc.
The determination of estimated POH at a selected flaw size may be a directly measured or observed value between 0 and 1. For a single trial, a “miss” is equal to 0 and a “hit” is equal to 1. Knowledge of an estimated POH yields a measure of the lower confidence bound, or PL. This process is statistically referred to as “observation of occurrences” and is distinct from use of functional forms that predict POD.
Traditionally, binomial distributions have been used for determining POD by direct observation of occurrences. Conventional binomial methodologies use a selection of arrangements for grouping flaws of similar characteristics. These approaches have led to the general acceptance of using the 29 out of 29 (29/29) binomial point estimate method, in combination with validation that the POD is increasing with flaw size, in order to meet certain governmental requirements or standards, e.g., MSFC-STD-1249, NASA-STD-5009, or similar standards.